typing: set warn_unreachable

This makes mypy raise an error whenever it detects code which is
statically unreachable, e.g.

    x: int
    if isinstance(x, str):
        ... # Statement is unreachable  [unreachable]

This is really neat and finds quite a few logic and typing bugs.

Sometimes the code is intentionally unreachable in terms of types, e.g.
raising TypeError when a function is given an argument with a wrong
type. In these cases a `type: ignore[unreachable]` is needed, but I
think it's a nice code hint.

testing/test_pytester.py

@@ -23,7 +23,9 @@ def test_make_hook_recorder(testdir) -> None:
     recorder = testdir.make_hook_recorder(item.config.pluginmanager)
     assert not recorder.getfailures()
 
-    pytest.xfail("internal reportrecorder tests need refactoring")
+    # (The silly condition is to fool mypy that the code below this is reachable)
+    if 1 + 1 == 2:
+        pytest.xfail("internal reportrecorder tests need refactoring")
 
     class rep:
         excinfo = None